Re: [uclibc-ng-devel] [PATCH] Fix "pselect: Use linux pselect6 syscall when available"

Commit e3c3bf2b58 introduce use of pselect6, but has following disadvantages: * Use of userspace types in args67 structure - it breaks, for example, configs when 32-bit uClibc-ng compiled against 64-bit kernel. Syscall will always return EINVAL. We must use __kernel_* types and __SYSCALL_SIGSET_T_SIZE. * It have excess checks for NSEC_PER_SEC. Original code from select() implementation has struct timeval => struct timespec conversion, kernel select() syscall implementation do the same. But none of libc versions (glibc, eglibc, musl) I know, perform similar checks for pselect() - there is no structure fields conversions, just struct timespec through all the calls. To have such checks in uClibc-ng we need one example, at least. * It is possible to avoid extra userspace reads from kernel code if sigmask == NULL. I suggest to do it, for a few bytes cost. * Commit didn't add test case to testsuite. Signed-off-by: Leonid Lisovskiy &lt;lly.dev(a)gmail.com&gt; --- libc/sysdeps/linux/common/pselect.c | 67 ++++++++++++------------------------- test/unistd/tst-pselect.c | 51 ++++++++++++++++++++++++++++ 2 files changed, 73 insertions(+), 45 deletions(-) create mode 100644 test/unistd/tst-pselect.c diff --git a/libc/sysdeps/linux/common/pselect.c b/libc/sysdeps/linux/common/pselect.c index 3f1dd28..fbe85b7 100644 --- a/libc/sysdeps/linux/common/pselect.c +++ b/libc/sysdeps/linux/common/pselect.c @@ -31,55 +31,32 @@ static int __NC(pselect)(int nfds, fd_set *readfds, fd_set *writefds, const sigset_t *sigmask) { #ifdef __NR_pselect6 -#define NSEC_PER_SEC 1000000000L - struct timespec _ts, *ts = 0; - if (timeout) { - /* The Linux kernel can in some situations update the timeout value. - * We do not want that so use a local variable. - */ + /* The Linux kernel can in some situations update the timeout value. + * We do not want that so use a local variable. + */ + struct timespec _ts; + + if (timeout != NULL) { _ts = *timeout; + timeout = &_ts; + } + /* Note: the system call expects 7 values but on most architectures + we can only pass in 6 directly. If there is an architecture with + support for more parameters a new version of this file needs to + be created. */ + struct { + __kernel_ulong_t ss; + __kernel_size_t ss_len; + } data; - /* GNU extension: allow for timespec values where the sub-sec - * field is equal to or more than 1 second. The kernel will - * reject this on us, so take care of the time shift ourself. - * Some applications (like readline and linphone) do this. - * See 'clarification on select() type calls and invalid timeouts' - * on the POSIX general list for more information. - */ - if (_ts.tv_nsec >= NSEC_PER_SEC) { - _ts.tv_sec += _ts.tv_nsec / NSEC_PER_SEC; - _ts.tv_nsec %= NSEC_PER_SEC; - } - - ts = &_ts; + if (sigmask != NULL) { + data.ss = (__kernel_ulong_t) sigmask; + data.ss_len = __SYSCALL_SIGSET_T_SIZE; + + sigmask = (void *)&data; } - /* The pselect6 syscall API is strange. It wants a 7th arg to be - * the sizeof(*sigmask). However syscalls with > 6 arguments aren't - * supported on linux. So arguments 6 and 7 are stuffed in a struct - * and a pointer to that struct is passed as the 6th argument to - * the syscall. - * Glibc stuffs arguments 6 and 7 in a ulong[2]. Linux reads - * them as if there were a struct { sigset_t*; size_t } in - * userspace. There woudl be trouble if userspace and the kernel are - * compiled differently enough that size_t isn't the same as ulong, - * but not enough to trigger the compat layer in linux. I can't - * think of such a case, so I'm using linux's struct. - * Furthermore Glibc sets the sigsetsize to _NSIG/8. However linux - * checks for sizeof(sigset_t), which internally is a ulong array. - * This means that if _NSIG isn't a multiple of BITS_PER_LONG then - * linux will refuse glibc's value. So I prefer sizeof(sigset_t) for - * the value of sigsetsize. - */ - struct { - const sigset_t *sigmask; - size_t sigsetsize; - } args67 = { - sigmask, - sizeof(sigset_t), - }; - - return INLINE_SYSCALL(pselect6, 6, nfds, readfds, writefds, exceptfds, ts, &args67); + return INLINE_SYSCALL(pselect6, 6, nfds, readfds, writefds, exceptfds, timeout, sigmask); #else struct timeval tval; int retval; diff --git a/test/unistd/tst-pselect.c b/test/unistd/tst-pselect.c new file mode 100644 index 0000000..cab9451 --- /dev/null +++ b/test/unistd/tst-pselect.c @@ -0,0 +1,51 @@ +#include <stdio.h> +#include <stdlib.h> +#include <string.h> +#include <unistd.h> +#include <errno.h> +#include <signal.h> +#include <sys/types.h> +#include <sys/select.h> + +// our SIGALRM handler +void handler(int signum) { + (void)signum; + puts("got signal\n"); +} + +static int +do_test (void) +{ + int rc; + sigset_t wait_mask, mask_sigchld; + struct sigaction act; + + // block SIGALRM. We want to handle it only when we're ready + sigemptyset(&mask_sigchld); + sigaddset(&mask_sigchld, SIGALRM); + sigprocmask(SIG_BLOCK, &mask_sigchld, &wait_mask); + sigdelset(&wait_mask, SIGALRM); + + // register a signal handler so we can see when the signal arrives + memset(&act, 0, sizeof(act)); + sigemptyset(&act.sa_mask); // just in case an empty set isn't all 0's (total paranoia) + act.sa_handler = handler; + sigaction(SIGALRM, &act, NULL); + + // send ourselves a SIGARLM. It will pend until we unblock that signal in pselect() + printf("sending ourselves a signal\n"); + kill(getpid(), SIGALRM); + + printf("signal is pending; calling pselect()\n"); + rc = pselect(0, NULL, NULL, NULL, NULL, &wait_mask); + if (rc != -1 || errno != EINTR) { + int e = errno; + printf("pselect() returned %d, errno %d (%s)\n", rc, e, strerror(e)); + exit(1); + } + + return 0; +} + +#define TEST_FUNCTION do_test () +#include <test-skeleton.c> -- 1.8.5.6